All of the older BMWs used that separate left and right turn signal switch system since the beginning of time. Basically BMW did it the way they always did it and refused to get dragged along when (almost) everyone else used a single switch on the left handlebar. As someone used to Japanese bikes, BMW's old turn signal switch setup drove me NUTS on the 2007-ish F800ST that I had as a rental once upon a time.
I think BMW changed over to match everyone else when the S1000RR was introduced - they probably realised that if they wanted to get customers who previously bought other brands, they would have to get with the program and do this little thing the way everyone else was doing it.